The great majority of studies of visual search have suggested that behavior is either one of two types. In some cases, it appears that capacity is unlimited, stimuli are processed in parallel and attention is spread out over the entire visual field. In other cases, it appears that capacity is limited, stimuli are processed in series, and attention is narrowly focused on a single stimulus. Dual process theories have been used to explain why the type of stimulus materials, the level of practice and the consistency of the mapping of stimuli to responses determines which of the two types of behavior will be observed. Recently, it has been argued that the dual process theories are too extreme. The overall goal of the proposed research is to develop an alternative single process theory that can better explain the changes in behavior in visual search tasks that occur with changes in stimuli, practice and mapping. First, tests will be made of a model of central processing which assumes that the number of comparison channels varies in predictable ways with the type of task (an extension of Fisher's limited channel model). Second, tests will be made of changes in the size of the effective visual field which occur with changes in the type of task using a Stanford Research Institute Eye Tracker. These results will help determine whether the comparison channels are spatially arrayed over the visual field. Finally, tests will be made of the effect on performance of variations in the number of critical features to which attention must be paid. These results will suggest whether the actual function of the comparison channels is changing with changes in stimuli, practice and mapping. When fully articulated, the single process theory should provide a complete account of how the number of channels, the focus of attention, and the activity on each channel varies with the nature of the task. Finally, the above research has an applied significance. In particular, it may help determine the sources of the observed information processing deficits in schizophrenics.